1. Technical Field of the Invention
The present invention relates generally to electric generators. More particularly, the invention relates to a tandem alternator for a vehicle, which has a reduced length in the axial direction of a rotary shaft thereof and can effectively suppress magnetic leakage between power generation units arranged in tandem in the axial direction.
2. Description of the Related Art
A conventional tandem alternator 500 for a vehicle includes, as shown in FIG. 6, a single rotary shaft 501 and two power generation units 502 and 503 that are arranged in tandem in the axial direction of the rotary shaft 501.
The power generation unit 502 includes a field 502A and an armature 502B, and the power generation unit 503 includes a field 503A and an armature 503B. The fields 502A and 503A of the power generation units 502 and 503 are arranged in tandem on the rotary shaft 501; the armatures 502B and 503B are arranged in tandem in the axial direction of the rotary shaft 501 and surround the fields 502A and 503A, respectively.
With the above tandem arrangement, it is possible to make the alternator 500 compact while enabling the alternator 500 to produce two different voltages. That is to say, compared to a scheme of employing two alternators producing different voltages, both the manufacturing cost and the installation space can be significantly reduced. In addition, it is preferable for the tandem alternator 500 to produce a typical voltage of 12V and a higher voltage of 42V to meet different voltage requirements of electrical loads provided on the vehicle.
However, with the above tandem arrangement, the axial length of the alternator 500 (i.e., the length of the alternator 500 in the axial direction of the rotary shaft 501) becomes more than 1.5 times that of a typical alternator that includes only a single power generation unit. Thus, in installation of the tandem alternator 500 to an engine of the vehicle, it is difficult to suitably arrange the alternator 500 without causing interference between the alternator 500 and an intake or exhaust pipe of the engine.
Accordingly, it is desired to reduce the axial length of the tandem alternator 500. However, when the power generation units 502 and 503 are arranged close to each other in the axial direction of the rotary shaft 501 for the purpose of reducing the axial length of the alternator 500, magnetic leakage may occur between the power generation units 502 and 503.
More specifically, the tandem alternator 500 includes a front housing 504, a rear housing 505, and a plurality of bolts 506. The front and rear housings 504 and 505 are arranged in the axial direction of the rotary shaft 501 with the armatures 502B and 503B intervening therebetween. Each of the bolts 506 extends in the axial direction of the rotary shaft 501 to tightly connect the front and rear housings 504 and 505, thereby securely retaining the armatures 502B and 503B between the front and rear housings 504 and 505. Further, in terms of cost and tensile strength, it is desirable to use a magnetic material to make the bolts 506. However, when the bolts 506 are made of a magnetic material, magnetic leakage (i.e., leakage of magnetic flux) may occur between the power generation units 502 and 503 through the bolts 506. Consequently, it may become difficult to accurately and separately control the power generation units 502 and 503; in addition, the bolts 506 may be loosened by heat induced therein due to the leakage flux passing therethrough.
One approach to the above problems is to space the fields 502A and 502B of the power generation units 502 and 503 in the axial direction of the rotary shaft 501, as shown in FIG. 6, thereby preventing formation of a closed magnetic path that is composed of the filed 502A, the armature 502B, the bolts 506, the armature 503B, and the field 503B. As a consequence, the magnetic leakage between the two power generation units 502 and 503 can be suppressed. (Such an approach is disclosed, for example, in Japanese Utility Model First Publication S57-42565.)
Japanese Patent First Publication No. H5-308751 discloses an automotive tandem alternator, in which the two fields are also spaced in the axial direction of the rotary shaft with a cooling fan interposed therebetween. In addition, the outer diameters of the armatures are made different from each other, so as to further reliably prevent formation of a closed magnetic path as described above.
Japanese Patent First Publication No. H11-98789 discloses an automotive tandem alternator, in which the front-side and rear-side armatures are respectively secured by a plurality of front-side bolts and a plurality of rear-side bolts. The front-side bolts are spaced from the rear-side bolts in the axial direction of the rotary shaft, so as to prevent formation of a closed magnetic path that is composed of the front-side field, the front-side armature, the front-side bolts, the rear-side bolts, the rear-side armature, and the rear-side field.
However, using any of the above approaches disclosed in the prior art, axial lengths of tandem alternators are increased, rather than reduced, thus making it difficult to install the tandem alternators in narrow engine compartments.